Modular resistivity sensor for downhole measurement while drilling

ABSTRACT

A resistivity-measuring sensor disposable within a drillstring includes a sensor body having a longitudinal axis, wherein the sensor body is separable from and disposable in the drillstring at a radially offset distance from the longitudinal axis of the drillstring. The sensor further includes a transmitting antenna disposed along a length of the sensor body, a receiving antenna disposed along a length of the sensor body, and an electronics section contained within the sensor body for generating and receiving signals to and from the transmitting and receiving antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 14/307,293, filedJun. 17, 2014, and allowed Jan. 17, 2017, which claims priority under 35U.S.C. § 119(e) to U.S. Provisional Application No. 61/836,577 filedJun. 18, 2013, which is incorporated herein by reference in itsentirety.

FIELD

Embodiments disclosed herein relate to, for example, apparatus andmethods for making electromagnetic resistivity measurements. Moreparticularly, embodiments disclosed herein relate to a resistivitymeasuring apparatus and methods employing a modular resistivity sensor.

BACKGROUND AND SUMMARY

Well logging, also known as borehole logging, is the practice of makinga detailed record (a well log) of the geologic formations penetrated bya borehole. Resistivity logging is a method of well logging that worksby characterizing the rock or sediment in a borehole by measuring itselectrical resistivity. Resistivity is a fundamental material propertywhich represents how strongly a material opposes the flow of electriccurrent. Most rock materials are essentially insulators, while theirenclosed fluids are conductors. Hydrocarbon fluids are an exception,because they are almost infinitely resistive. When a formation is porousand contains salty water, the overall resistivity will be low. When theformation contains hydrocarbons, or contains very low porosity, itsresistivity will be high. High resistivity values may indicate ahydrocarbon bearing formation.

In one aspect, embodiments disclosed herein relate aresistivity-measuring sensor disposable within a drillstring. The sensorincludes a sensor body having a longitudinal axis, wherein the sensorbody is separable from and disposable in the drillstring at a radiallyoffset distance from the longitudinal axis of the drillstring. Thesensor further includes a transmitting antenna disposed along a lengthof the sensor body, a receiving antenna disposed along a length of thesensor body, and an electronics section contained within the sensor bodyfor generating and receiving signals to and from the transmitting andreceiving antennas.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an embodiment of a modular resistivity sensor.

FIG. 2 illustrates an embodiment of a sensor body of the modularresistivity sensor depicted in FIG. 1.

FIG. 3 illustrates an embodiment of a sensor cover of the modularresistivity sensor depicted in FIG. 1.

FIG. 4 illustrates an embodiment of a modular resistivity sensorincluding a calibrating antenna.

FIG. 5 illustrates an embodiment of a modular resistivity sensorassembly within a tool body.

FIG. 6 illustrates a flowchart showing a method of using the modularresistivity sensor.

FIG. 7 illustrates a representative computer model showing azimuthalresolution by a modular resistivity sensor placed parallel to aformation bed.

FIG. 8 illustrates a graph showing the differences in the attenuationmeasurements (in dB) in the presence of a bed boundary separating a1-ohmm bed from a 100-ohmm bed with the sensor in the 100-ohmm bed.

FIG. 9 illustrates an embodiment of a modular resistivity sensorcombined with other sensors in a drillstring.

DETAILED DESCRIPTION

A downhole resistivity measuring tool suitable for use in any downholeenvironment is disclosed. A drill bit is secured to the lower end of thedrillstring or drill tool body for drilling a rock formation. Themeasuring tool includes a modular resistivity measurement sensor. Themodular resistivity sensor includes a sensor body with a longitudinalaxis. At least part of the body may be made of non-conducting materialsuch as rubber, PEEK, fiberglass, ceramic, or others. The sensor bodyprovides no fluid conduit within for passage of drilling fluids (e.g.,drilling mud). At least one transmitting coil antenna and at least onereceiving coil antenna are disposed in/about the sensor body. Thetransmitting and receiving coil antennas each include one or multipleturns of wire wound about the sensor body. At least one coil antennagenerates a magnetic moment in a direction parallel to the longitudinalaxis of the sensor body. The receiving coil antenna may generate amagnetic moment in a direction parallel to or at an angle (e.g.,orthogonal) with respect to the longitudinal axis of the sensor body.The sensor body may further include an electronics section forgenerating and receiving electromagnetic signals to and from thetransmitting and receiving antennas. The electronics section ispreferably contained in the sensor body, but it may also be contained ata separate location.

Resistivity measuring tools use an electric coil to generate analternating current loop in the formation by induction (e.g., firing atransmitting coil). The alternating current loop, in turn, induces avoltage signal in a receiving coil located elsewhere in the tool. Thevoltage signal induced in the receiving coil is related to a parameterof the formation. Multiple transmitting and receiving coils may be usedto focus formation current loops both radially (depth of investigation)and axially (vertical resolution).

FIG. 1 illustrates an embodiment of a modular resistivity sensor 100.The sensor 100 includes a sensor body 101 having a longitudinal axis103, and one or more coil antennas wound about the sensor body 101. Asshown, a transmitting coil antenna 102 and two receiving coil antennas104 are wound about the sensor body 101. Any number of coil antennas maybe used. For example, although the exemplary sensor shown employs onlyone transmitting coil antenna, it is possible to use multipletransmitting coil antennas. For instance, a second transmitting coilantenna may be added to the sensor body on the other side of thereceiving coil antenna array. The two transmitting coil antennas may besymmetrical or asymmetrical with respect to the receiving coil antennas.The two transmitting coil antennas may be energized independently orjointly. The receiving coil antennas 104 may be spaced by at least twoinches, or at least three inches, or at least six inches, or at leastten inches, or greater. The transmitting coil antenna 102 may be spacedfrom the receiving coil antennas 104 by a few inches to a few feet, suchas at least three inches, or at least ten inches, at least one foot, atleast two feet, or at least three feet, or greater.

An electronics section 110 may be included in the sensor body 101 fortransmitting or receiving electromagnetic energy to and from the coilantennas. The electronics section 110 may be powered internally (e.g.,batteries) or externally by another tool sub. The electronics section110 may be equipped with a micro controller and an optional memorydevice. The acquired data may be stored in the memory and/or transmittedto a remote location (e.g., a nearby measurement-while-drilling sub) fortransmission to the surface.

FIG. 2 illustrates a perspective view of the sensor body 101 depicted inFIG. 1. In one embodiment, the sensor body 101 may be made of steel orother metal. The sensor body 101 may be cylindrical, but also may beother shapes. The diameter of the sensor body 101 may be at leastone-half inch, or at least one inch, or at least two inches, or at leastfour inches, or greater. Preferably, the sensor body diameter is betweenapproximately one and two inches. Antenna grooves 105 may be formed inan outer surface around a circumference of the sensor body 101 for eachcoil antenna. The antenna grooves 105 may be any depth, althoughpreferably around 0.25 inches, or at least 0.50 inches, or at least 0.75inches. An insulation layer of any thickness may be deposited in theantenna grooves 105, over which the coil antenna wire is wound. Theelectronics is housed in an electronics pocket, and the electronicssection may be sealed within the pocket from ambient pressure by apocket cover 107 equipped with O-rings. The pocket cover 107 is securedto the sensor body 101 by bolts, screws, rivets, or other fasteningmeans (not shown). The communication wires from the coil antennas to theelectronics section 110 may enter the electronics pocket throughpressure-sealed feed-thrus (not shown). The feed-thrus are preferablyplaced near the respective coil antennas.

FIG. 3 illustrates a sensor sleeve 112 in the shape of a hollow tubeinto which the sensor body and coil antennas may be inserted forprotection of the coil antenna wire. The sleeve 112 may be in the shapeof a cylinder and may be made of non-conducting material (e.g., plasticor rubber) or conducting material (e.g., steel). For a steel tube,windows or slots 109 may be formed through an outer wall of the tube inlocations where coil antennas reside so that electromagnetic energy canescape or enter the tube. For maximum transmission of electromagneticenergy through the tube, the slots may be substantially aligned alongthe direction of magnetic moments generated by respective coil antennasunderneath. For example, for a coil antenna generating a magnetic momentparallel with a longitudinal axis of the sensor body 101, the slots 109may be substantially aligned parallel to the longitudinal direction ofthe sensor body.

Although the coil antenna grooves 105 and the electronics pocket 107 areshown to be deposited within the same sensor body 101, in alternativeembodiments, the sensor body may include two sub-bodies, one for thecoil antennas and the other for the electronics section. The coilantenna sub-body may be made completely out of non-conducting materialsuch as rubber, PEEK, fiberglass, or ceramic. In this case, theinsulation layer in each antenna groove may or may not be used withoutaffecting the transmission or reception of electromagnetic energy byeach antenna. If needed, the two sub-bodies may be connected to eachother by bolts, screws, or other fasteners to form a single sensor body.

FIG. 4 illustrates a further embodiment of a modular resistivity sensor100. The sensor 100 includes a sensor body 101 having a longitudinalaxis 103, and one or more coil antennas wound about the sensor body 101.As shown, a transmitting coil antenna 102 and two receiving coilantennas 104 are wound about the sensor body 101. The sensor 100 alsoincludes a calibrating coil antenna 106 wound about the sensor body 101.The calibrating coil antenna 106 is disposed between the two receivingcoil antennas 104. Preferably, the calibrating coil antenna 106 isequally spaced between the receiving antennas 104, but may also beunequally spaced. Methods of using the calibrating antenna are describedbelow.

FIG. 5 illustrates an embodiment of the modular resistivity sensor 100assembled within a bottomhole assembly (BHA) or tool body 50. The BHA 50has a longitudinal axis 51. The modular resistivity sensor 100 may bedisposed near the outer surface of the tool body 50. In one embodiment,a radially outermost surface of the modular resistivity sensor is at adistance from the longitudinal axis of the tool body substantially equalto or less than an outer diameter of the tool body. That is, no part ofthe modular resistivity sensor 100 protrudes or extends radially outwardbeyond the diameter of the tool body outer surface. Therefore, in theinstance that the tool body or drillstring include any type of componentat a radial distance beyond an outer surface of the tool body 50 (e.g.,a stabilizer), the modular resistivity sensor is disposed on the toolbody at a different location from the component (e.g., stabilizer), andnot on the component. That is, the modular resistivity sensor is notdisposed on a stabilizer, nor is it required to be. A sensor pocket 52extending radially inward from the outer surface is formed in the toolbody 50. The sensor pocket 52 may be any shape and may be formed in agroove that is cut in the tool body 50, or formed directly in the outersurface. For tool face reference, the modular resistivity sensor 100 maybe aligned to a scribe line on the tool body surface (not shown). Wireways may extend within the tool body 50 for power and/or signalcommunication between the modular resistivity sensor 100 and otherelectronics units within the tool body or drillstring.

A sensor cover 54 may be placed over the installed modular resistivitysensor 100 and attached to the tool body 50 to prevent the sensor 100from being damaged during the drilling process. The sensor cover 54 maybe made of abrasion-resistant steel, such as stellite, or othermaterials. The sensor cover 54 further includes slots or windows 56extending through an outer wall. For maximum transmission ofelectromagnetic energy across the sensor cover 54, the slots 56 may besubstantially aligned along the direction of magnetic moments generatedby respective coil antennas underneath. For example, for a coil antennagenerating a magnetic moment parallel with a longitudinal axis 51 of thetool body 50, the slots 56 may be substantially aligned parallel to thelongitudinal axis 51 of the tool body 50. Further, the slots 56 may befilled with non-conducting material such as rubber, PEEK, fiberglass, orceramic. In those instances when the modular resistivity sensor ishoused within a tube (shown in FIG. 3), preferably the tube slots 109are substantially aligned with the sensor cover slots 56.

Methods of using the modular resistivity sensor include measuringformation resistivity of a formation, making azimuthal resistivitymeasurements, and using the sensor with other sensors and downholetools. Measuring formation resistivity includes applying or generating asinusoidal electromagnetic wave of current to the transmitting coilantenna. The frequency of the sinusoidal wave may be between 100 kHz and1 GHz. A voltage signal is measured independently from each receivingcoil antenna, which may be expressed as V₁ and V₂, respectively. Avoltage signal difference may be calculated from V₁ and V₂ as:

V=V ₂ −αV ₁  (Equation 1)

where α is a scaling factor. The value of α may be chosen so that thevoltage signal difference V becomes zero when the measurement is takenin air. Voltage is a complex quantity having both in-phase andout-of-phase components, both of which may contain information about theformation resistivity. For example, when using the in-phase component ofV, R_(e)(V) to derive an apparent formation resistivity, the followingequation is used:

$\begin{matrix}{R_{\alpha} = {k\frac{R\;{e(V)}}{I}}} & ( {{Equation}\mspace{14mu} 2} )\end{matrix}$

where k is a proportionality factor. A value of k may be chose using anumerical model. In the model, a tool body containing the resistivitysensor is placed in a wellbore centered about the axis of a wellbore.The formation is assumed to be uniform with a resistivity value of R_(t)and the mud resistivity is the same as R_(t). The voltage signaldifference V for a given driving current I applied to the transmittingcoil antenna may be calculated. Finally, k may be calculated as:

$\begin{matrix}{k = {R_{t}\frac{t}{{Re}(V)}}} & ( {{Equation}\mspace{14mu} 3} )\end{matrix}$

It is also possible to use the ratio of V₂ to V₁ to derive a parameterof the surrounding formation. This may be done by defining two newquantities, attenuation (“AT”) and phase difference (“PD”), defined as:

$\begin{matrix}{{AT} = {{- 20}\mspace{11mu}\log\;{\frac{V_{2}}{V_{1}}}}} & ( {{Equation}\mspace{14mu} 4} ) \\{{PD} = {\tan^{- 1}( \frac{V_{2}}{V_{1}} )}} & ( {{Equation}\mspace{14mu} 5} )\end{matrix}$

Surrounding medium (e.g., drilling mud) and free space may contribute tothe AT and PD quantities, and preferably such contributions to thequantities are discarded by performing an “air hung” calibration. Thatis, both AT and PD quantities are measured with the sensor hung in air,away from any conductors that may interfere with the measurement. Theair hung measurements are then subtracted from subsequent downholemeasurements as follows:

ΔT*=AT−AT^(air)  (Equation 6)

PD*=PD−PD^(air)  (Equation 7)

The above discussion focuses on a single transmitting antenna. In casetwo transmitting antennas are used, average attenuation and phasedifference measurements may be produced from the individual transmittingantennas. It has been well known that the average attenuation and/orphase difference measurement helps remove electronic noises and boreholeeffects on the measurement.

Apparent resistivities may be derived from both AT* and PD* using thefollowing equations:

R _(α) ^(AT) =k ^(AT)AT*  (Equation 8)

R _(α) ^(PD) =k ^(PD)PD*  (Equation 9)

where k^(AT) and k^(PD) are proportionality factors. Methods of findingthese factors are well known and will not be detailed here. Forinstance, they can found through numerical modeling.

Measurements taken as described above may be subject to errors,including those from electronics noises, thermal drifts in theelectronics, and deformation and/or material properties changes of theantennas. Random errors may be removed or suppressed by time averaging.However, systematic errors, such as those from thermal drifts andantenna deformation, may not be reduced by time averaging. In thisinstance, compensating for systematic errors may include using acalibrating device, such as the calibrating antenna shown in FIG. 4. Acurrent is driven to the calibrating antenna at a desired frequency andthe attenuation and phase difference between the receiving antennas ismeasured as follows:

$\begin{matrix}{{AT}^{Cal} = {{- 20}\mspace{11mu}\log{\frac{V_{2}^{Cal}}{V_{1}^{Cal}}}}} & ( {{Equation}\mspace{14mu} 10} ) \\{{PD}^{Cal} = {- {\tan^{- 1}( \frac{V_{2}^{Cal}}{V_{1}^{Cal}} )}}} & ( {{Equation}\mspace{14mu} 11} )\end{matrix}$

where the subscripts “1” and “2” have the same meaning as above. Next,the attenuation and phase difference measurements from equations (10)and (11) are subtracted from the air-calibrated measurements as follows:

AT**=AT*−AT_(Cal)  (Equation 12)

PD**=PD*−PD^(Cal)  (Equation 13)

Apparent formation resistivities may then be calculated from AT** andPD** as in equations (8) and (9).

FIG. 6 illustrates a flowchart showing a method of using the modularresistivity sensor in accordance with the above description. The methodincludes firing a transmitting antenna (Step 602), calculatingattenuation and phase difference from induced voltage measuredindependently in receiving antennas (Step 604), and calibrating thecalculated attenuation and phase difference using air hung measurements(Step 606). The method further includes firing a calibrating antennalocated between receiving antennas (Step 608), calculating attenuationand phase difference from induced voltage measured independently inreceiving antennas while firing the calibrating antenna (Step 610), andsubtracting the calculated attenuation and phase difference from theprevious air hung calibrated measurements (Step 612). Finally, themethod includes calculating apparent formation resistivity fromattenuation and phase difference (Step 614).

In other embodiments, a calibrating signal generator may be used. Thesignal generator generates a calibrating signal (e.g., voltage) at adesired frequency and feeds the signal to the electronics that takemeasurements from the two receiving antennas. The calibratingattenuation and phase difference are then calculated in the mannerdiscussed above. In this case, the calibrating attenuation and phasedifference may account for variations in the receiving electronics butnot necessarily in the receiving antennas because the receiving antennasmay be bypassed.

Methods of using the modular resistivity sensor also include makingazimuthal resistivity measurements. A tool face sensor records the toolface angle as the tool body rotates and the modular resistivity sensormakes azimuthal measurements of formation resistivity. The tool facesensor may be a magnetometer, an accelerator, a gyro scope or otherknown tool face sensors. The resistivity measurements taken by themodular resistivity sensor may then be correlated with the tool faceangle measurements to produce a resistivity image as a function of toolface and a function of wellbore depth.

FIG. 7 illustrates a computer model for azimuthal resolution of theside-mounted modular resistivity sensor placed parallel to a formationbed boundary. ‘T’ indicates the transmitting antenna and ‘R1’ and ‘R2’indicate receiving antennas. Both attenuation and phase difference aremeasured between the two receiving antennas. The front side of themodular sensor is defined as one facing the bed boundary and the backside faces the opposite direction. Differences in the attenuation and/orphase difference measurements between the front and the back sides ofthe modular sensor indicates the azimuthal resolution. The larger thedifference, the better azimuthal resolution the sensor provides.

FIG. 8 illustrates a graph showing the differences in the attenuationmeasurements (in dB) in the presence of a bed boundary separating a1-ohmm bed from a 100-ohmm bed with the sensor in the 100-ohmm bed. Inthis example, a tool body having a diameter of five inches, atransmitting coil antenna spaced eight inches from a center of thereceiving coil antennas, and receiving coil antennas spaced four inchesapart was used. As shown, azimuthal resolution of the sensor improveswith increasing frequency, and decreases as the distance to the bedboundary increases. Also, a front side of the sensor measures higherattenuation than the back side, which is expected because the front sidefaces the more conductive bed. Hence, by measuring the tool face anglescorresponding to the front and the back sides, it is possible todetermine the azimuthal direction of the bed boundary relative to thetool.

Azimuthal resistivity measurements may also be taken without tool bodyrotation. To do so, multiple modular resistivity sensors may beazimuthally-spaced about a circumference of the tool body. The modularsensors are spaced apart by known tool face angles. A representativechoice of the azimuthal separation angle may be 90 degrees, or at least30 degrees, or at least 45 degrees. The multiple modular sensors may becontrolled by a micro-controller to take measurements independently.

FIG. 9 illustrates an embodiment of the modular resistivity sensor 100combined with other sensors in a drillstring 10. The exemplaryembodiment shown may be used for resistivity measurement-while-drilling(“MWD”). The drillstring 10 includes a drill bit 15 on a distal end anda modular resistivity sensor 100 located behind the drill bit 15. An MWDsub 35 including a tool face measurement sensor 40 is located behind thedrill bit 15 spaced apart from the modular resistivity sensor by a mudmotor 25 and/or other downhole tools (e.g., bent sub 20, LWD tool string30). The modular resistivity sensor 100 may measure azimuthalresistivity of the formation while the tool rotates and the sensor 40may measure tool face data. The azimuthal resistivity measurements arethen transmitted (shown by dashed lines indicating communication) to theMWD sub 35 for further transmission to the surface. In otherembodiments, azimuthal resistivity may be measured without a tool facesensor at the bit. To do so, the modular resistivity sensor 100 and theMWD sub 35 are first synchronized in time (e.g., at surface). Oncedownhole, the modular resistivity sensor 100 takes resistivitymeasurements while the MWD sub 35 measures tool face anglesindependently. The resistivity measurements with time stamps aretransmitted through short-hop telemetry to the MWD sub 35 where the timestamps are converted to tool face stamps.

In another embodiment, a modular resistivity sensor is combined withanother resistivity sensor (e.g., modular or non-modular) to makeresistivity measurements. A modular resistivity sensor, having at leastone transmitting antenna and/or at least one receiving antenna, isdirectly at or proximate to the drill bit. The second or additionalresistivity sensor is located farther behind the drill bit than themodular resistivity sensor, and spaced apart from the modularresistivity sensor by other downhole components (e.g., a mud motor and abent sub). The second resistivity sensor has at least one transmittingantenna and at least one receiving antenna. In operation, thetransmitting antenna on the second resistivity sensor sends anelectromagnetic signal to the surrounding formation which is detected bythe receiving antenna in the modular resistivity sensor. The receivedsignal may be processed locally by a processor at the bit or transmittedvia short-hop telemetry to the second resistivity sensor for processing.

In making an azimuthal resistivity measurements, tool face angles may bemeasured with a sensor located either at the bit or near the secondresistivity sensor. In case the portion of the BHA containing the secondresistivity sensor does not rotate or does not rotate at the same speedas the modular resistivity sensor does, the tool face sensor may belocated at or proximate to the drill bit or in any portion of the BHAthat rotates at the same speed as the modular resistivity sensor doesand the modular resistivity sensor has at least one transmitting and/orreceiving antenna whose magnetic moment is generated in a direction notparallel to the longitudinal axis of the tool sub.

The claimed subject matter is not to be limited in scope by the specificembodiments described herein. Indeed, various modifications of theinvention in addition to those described herein will become apparent tothose skilled in the art from the foregoing description. Suchmodifications are intended to fall within the scope of the appendedclaims.

What is claimed is:
 1. A resistivity-measuring sensor disposable withina tool body, the sensor comprising: a sensor body having a longitudinalaxis, wherein the sensor body is separable from and at least partiallydisposable within a tool body; a transmitting antenna disposed along alength of the sensor body; a receiving antenna disposed along a lengthof the sensor body; and an electronics section configured to receivesignals from the transmitting antenna.
 2. The sensor of claim 1, whereinat least one antenna is configured to generate magnetic moments parallelwith the longitudinal axis of the sensor body.
 3. The sensor of claim 1,wherein at least one antenna is configured to generate magnetic momentsmisaligned with the longitudinal axis of the sensor body.
 4. The sensorof claim 1, further comprising a calibrating device configured tocalibrate signals generated from the receiving antennas.
 5. The sensorof claim 4, wherein the calibrating device comprises a signal generatorconfigured to generate a voltage signal at a desired frequency.
 6. Thesensor of claim 4, wherein the calibrating device comprises an antennaconfigured to generate a voltage signal at a desired frequency in thereceiving antennas.
 7. The sensor of claim 4, wherein the calibratingdevice is substantially equally spaced between a pair of receivingantennas.
 8. The sensor of claim 1, wherein at least part of the sensorbody comprises a non-conducting material.
 9. The sensor of claim 1,further comprising circumferential grooves in an outer surface of thesensor body, each groove having an insulating layer deposited therein onwhich the antennas are wound.
 10. The sensor of claim 1, furthercomprising a sensor cover that fits over the sensor body, wherein atleast part of the sensor cover comprises a non-conducting material. 11.The sensor of claim 10, wherein the sensor cover further comprises oneor more slots aligned substantially along a direction of magneticmoments generated by respective antennas underneath.
 12. The sensor ofclaim 1, wherein a radially outermost surface of the sensor body is at adistance from the longitudinal axis of the drillstring substantiallyequal to or less than an outer diameter of the drillstring.
 13. Thesensor of claim 1, further comprising a tool face measurement sensorconfigured to measure the angular position of the drillstring relativeto the wellbore.
 14. The sensor of claim 1, further comprising multiplesensor bodies disposed in drillstring pockets, wherein electromagneticsignals are transmitted and received at multiple antennas on themultiple sensor bodies.
 15. The sensor of claim 1 which is configuredsuch that when the tool body rotates, transmitting and receivingpatterns from the antennas rotate with the tool body.
 16. A resistivitysensor comprising: at least one transmitting antenna; at least onereceiving antenna; a calibrating antenna located longitudinally betweenthe at least one receiving antenna and an electronics section containedwithin a body of the resistivity sensor; wherein the sensor isconfigured to be disposed within and separable from a tool body.
 17. Theresistivity sensor of claim 16 wherein the sensor is configured to beradially offset from a longitudinal axis of the tool body.
 18. Theresistivity sensor of claim 16, wherein at least one antenna isconfigured to generate magnetic moments parallel with a longitudinalaxis of the sensor body.
 19. The resistivity sensor of claim 16, whereinat least one antenna is configured to generate magnetic momentsmisaligned with the longitudinal axis of the sensor body.
 20. Theresistivity sensor of claim 16 further comprising a sensor cover thatfits over the sensor body, wherein at least part of the sensor covercomprises a non-conducting material.